Surface acoustic wave devices (SAW devices) utilize elastic waves generated along the surface of a piezoelectric layer to perform a variety of useful functions. A typical SAW device comprises a pair of surface wave transducers disposed on a smooth surfaced piezoelectric substrate. The transducers are typically interdigital transducers (IDTs), each comprising a pair of alternating, interpenetrating finger-shaped electrodes. The piezoelectric substrate is typically lithium niobate, lithium tantalate or quartz. In operation, a transducer is driven by a radio frequency source (as by applying the source between the electrodes of an IDT) and a surface acoustic wave is launched across the substrate to the second transducer where it can be picked up as an RF signal.
Surface acoustic wave devices can be used in a variety of applications including filtering, provision of signal delay and generation of high frequency oscillations. Such devices and their applications are described in greater detail in the following publications, all of which are incorporated herein by reference: C. Hartman et al., Proceedings of the 37th Annual Frequency Control Symposium, p. 354 et seq. (1993); R. Weigel et al., Proceedings of the 1996 IEEE Microwave Theory and Technology Conf., Paper No. WE1A-5, p.413 (June 1996); Y. Taguchi et al., Proceedings of the 1996 IEEE Microwave Theory and Technology Conf., Paper No. WE1A-5, p:417 (June 1996); S. Shikata et al., Proceedings of the Third International Conf. on Applications of Diamond Films and Related Materials, NIST Special Publication No. 885, p. 29 (1995) and M. Feldmann et al., Surface Acoustic Waves for Signal Processing, Chap. 4 (Artech House, 1989).
One important limitation on SAW devices for telecommunications applications is that they are difficult to tune. A SAW filter, for example, is inherently a narrow bandwidth device with a maximum filtering response at a frequency f=v.sub.p /.lambda. where v.sub.p is the acoustic phase velocity of the sound transmitting medium and .lambda. represents the geometric spacing of the IDT fingers. Since v.sub.p and .lambda. in a conventional SAW device are fixed, such devices operate at a fixed frequency and lack the tunability required for many telecommunications applications. Accordingly, there is a need for a tunable SAW device.